Method of calibrating a vapor detector

ABSTRACT

A highly selective, sensitive, fast detection system and method are disclosed for detecting vapors of specific compounds in air. Vapors emanating from compounds such as explosives, or stripped from surfaces using heat and suction from a hand-held sample gun, are collected on surfaces coated with gas chromatograph (GC) material which trap explosives vapors but repel nitric oxide, then are desorbed and concentrated in one or more cold spot concentrators. A high speed gas chromatograph (GC) separates the vapors, after which specific vapors are decomposed in two pyrolyzers arranged in parallel and the resulting nitric oxide is detected. A low temperature pyrolyzer with silver produces NO from nitramines or nitrite esters; a high temperature pyrolyzer decomposes all explosives vapors to permit detection of the remaining explosives. Also disclosed is a series arrangement of pyrolyzers and gas chromatographs and an NO detector to time-shift detection of certain vapors and facilitate very fast GC analyses. The use of hydrogen as a carrier gas, plus unique collectors and concentrators, high speed heaters, NO detectors, and very fast, temperature-programmable GC&#39;s enhance selectivity, sensitivity and speed of detection.

This invention was made with Government support under Contract No2038-563371 awarded by the Department of State. The Government hascertain rights in this invention.

BACKGROUND AND OBJECTS OF THE INVENTION

This invention relates to selective detection of specific compounds andin particular to a simple method of calibrating or checking operation ofa vapor detector such as an explosives vapor detection system.

Detection of explosives carried by persons or concealed in buildings,airplanes, cars or other locations can be vital to prevention ofinjuries and damage to property. However, detection by direct searchingis quite costly and time-consuming, can at times be dangerous, and canalso be susceptible to error. Thus, it is desirable to detect explosivessomewhat indirectly, as by their presence in very small amounts ofvapors in air or other gases which have been in contact with explosivesin solid or liquid form.

To be effective, devices for detecting vapors of selected compounds suchas explosives in air must fulfill several requirements. They must, ofcourse, be reliable. Also, they need to be highly sensitive in order todetect the minute quantities (parts per quadrillion (10¹⁵)or less)present in vapors and which in turn may indicate the presence of muchlarger quantities of the compounds. It is essential that explosivesdetection systems be very selective so as to prevent or minimize falsealarms which would result from detection of compounds which are notexplosives, and yet be highly reliable so that no explosives present areoverlooked or not detected. In certain applications, such as screeningpersons for possession of explosives, detectors must operaterapidly--they must determine, essentially in real time, whetherexplosives are present--and they should also be as non-intrusive aspossible. For many situations, it is important that the detectoridentify the specific explosive detected. Other characteristics whichmay be important in an explosives detector are that it be portable,rugged, and able to function in harsh environments.

Various systems are known for detecting specific compounds such asexplosives, but none have provided the combination of selectivity,sensitivity, reliability, and rapid response needed for an effective andreliable detector. Systems such as electron capture detectors, massspectrometers, ion mobility spectrometers, and nitric oxidechemiluminescence analyzers have been employed for detecting explosives,as have certain animals (notably dogs). The systems may performsatisfactorily if provided with high or moderate levels of certainexplosives vapors and if allowed ample time for analysis. However, theygenerally are slow and also fail to provide the selectivity todistinguish explosives from various other compounds, particularlynitrogen-containing compounds, whose vapors may be present along withthe explosives. The selectivity of such systems decreases as theconcentration of explosives decreases and is a significant drawback indetection of low levels of explosives. As a result, non-explosives suchas halogenated solvents, nitorsamines, perfumes, nitrogen oxides(NO_(x)), and phthalates interfere with, and may give false readingsinstead of, accurate detection of explosives.

It is an object of the invention to provide a simple method ofcalibrating a vapor detector.

It is an object of the invention to provide a method of calibrating avapor detector wherein vapors of specific compounds may readily begenerated in remote locations.

It is an object of the invention to provide a method of rapidlycalibrating or checking response of a vapor detector which utilizespre-packaged, sealed samples of treated sorbent material as a source ofvapors and avoids the need to inject liquids into a gas stream.

It is an object of the invention to provide a method of calibrating orchecking response of an explosives vapor detector which utilizes a vaporcollection technique similar to that employed by the vapor detector.

SUMMARY OF THE INVENTION

The invention is a simple, effective method of calibrating a vapordetector, particularly a detector of vapors of specific organiccompounds such as explosives or certain drugs. According to the method,a solution of one or more specific compounds which the detector isdesigned to detect is applied to a sorbent material and (optionally) thesorbent material is sealed in a container. When a calibration or systemcheck of the vapor detector is desired, the sorbent material is heatedand vapors of the specific compounds from the sorbent material are drawnin an air sample through a collector and trapped on special surfaces.The trapped vapors are then analyzed by the vapor detector to determineits response and verify its proper operation.

A preferred technique includes spraying a mixture of specific compounds,such as explosives, on a paper towel or similar material, then sealingthe towel in a air-tight container such as a foil pouch. Operation ofthe vapor detector is checked by unsealing the container, heating thetowel with lamps of a hand-held sample gun, and drawing air over thetowel and through a cartridge-like collector in the bore of the samplegun. Vapors trapped in coated surfaces of the collector are thenanalyzed in the vapor detector. Preferablin in a series of stepsincluding desorbing vapors from the collector into a carrier gas,concentrating the vapors, gas chromatographically separating, thendecomposing, the vapors to produce nitric oxide, and detecting thenitric oxide produced. Proper operation of the detector is confirmed byidentifiable peaks on a chromatogram for each specific compound whichhad been applied to the paper towel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an vapor detection system showing itsprincipal subsystems.

FIG. 2 is a side view of a sample collector having a bundle of coatedtubes for trapping explosives vapors.

FIG. 3 is a cross-sectional view of the sample collector taken along theline 3--3 of FIG. 2.

FIG. 4 is a single tube of a collector enlarged to show various coatingson its surfaces.

FIG. 5 is an end view of a sample collector having a metal bundle ofcoated tubes in contact with a spiral-wound metal foil.

FIG. 6 is an end view of a sample collector having a bundle of coatedtubes in contact with a metal foil wound in a double spiral.

FIG. 7 is an end view of a sample collector having a bundle of coatedtubes in contact with a spiral wound metal foil which is corrugated topermit increased packing density.

FIG. 8 is an end view of a spiral wound sample collector utilizing ametal ribbon as a substrate.

FIG. 9 is an end view of a sample collector which includes a porous fritas the substrate.

FIG. 10 is a side view of a hand-held sampler or sample gun forcollecting vapors showing a collector assembly within the gun.

FIG. 11 is a front end view of the sample gun of FIG. 9.

FIG. 12 is a side view of a sample collector/desorber assembly of thedetector system with a collector assembly held therein.

FIG. 13 is a front view, partly broken away, of the samplecollector/desorber assembly of FIG. 12 illustrating its front mountingplate.

FIG. 14 is a side view of a cold spot for concentrating vapors followingtheir collection.

FIGS. 15(a) and 15(b) are an end view and a side view respectively, of acold block and cooler assembly for two cold spot concentrators.

FIGS. 16(a) and 16(b) are schematic diagrams of a detector system withtwo cold spots and illustrating conduits and valving associated withflows of gases and vapors during sampling and analysis.

FIG. 17 is a side view of a pyrolyzer of the detection system.

FIG. 18 is a view of an ozone-based chemiluminescence NO detectorsuitable for use in the vapor detection system.

FIG. 19 is a sketch of a chromatogram which would result from analysisin an NO detector of the products of decomposition of a gas sample in alow temperature pyrolyzer.

FIG. 20 is a sketch of a chromatogram which would result from analysisin an NO detector of the products of decomposition in a high temperaturepyrolyzer.

FIG. 21 shows several chromatograms resulting from analysis in an NOdetector of the products of decomposition of gas samples of knowncomposition in a low temperature or high temperature pyrolyzer.

FIG. 22 is a block diagram of a vapor detection system which includestwo pyrolyzers and two gas chromatographs in a series-connectedarrangement.

FIG. 23 is a sketch of a timewise trace of output signals from adetection system such as that of FIG. 22.

FIGS. 24 and 25 are chromatograms of tests performed on a system similarto that of FIG. 16.

DETAILED DESCRIPTION

Detection of vapors such as explosives, as disclosed herein, is based ontrapping and concentrating small amounts of vapor given off byexplosives or stripped from surfaces contaminated by trace amounts ofexplosives, decomposing the concentrated vapors to produce nitric oxidegas (NO), and then detecting the NO from the decomposed vapors. It isessential to and a major feature of the vapor detection system that itis highly selective. Even when used to monitor air samples containingminute quantities of explosives (1 part in 10¹⁴ or less) andconsiderably higher concentrations of nitrogen oxides and non-explosiveswhich could yield NO upon heating, the detector systematically avoidsresponse to all compounds which are not of interest and correctlyidentifies the explosives.

The vapors trapped and detected according to the techniques andapparatus disclosed herein may be in the gas phase, or adsorbed on small(microscopic) particles, or dissolved in aerosol droplets. The surfacesdescribed collect not only vapor in the gas phase but the particulatesand aerosol droplets as well, and the term "vapors" as used hereinrefers to vapors collected in any of these forms.

Explosives to be monitored all contain nitrogen, and most include one ormore nitrite (--NO₂) functional groups, typically attached to a carbon,nitrogen, or oxygen atom. Examples of explosives of interest aretrinitrotoluene (TNT), dinitrotoluene (DNT), nitroglycerin (NG),pentaerythritol tetranitrate (PETN), ethylene glycol dinitrate (EGDN),cyclotetramethylene tetranitramine (HMX), cyclo1,3,5-trimethylene-2,4,6-trinitramine (RDX), and water gels (ammoniumnitrate plus additives).

A preferred vapor detector system, shown schematically in FIG. 1,includes five subsystems which are connected in series and linked to amicroprocessor unit 28 containing one or more microprocessors whichcontrols their operation in rapid, highly selective and sensitivedetection of vapors, such as explosives vapors, in air samples. Theprincipal subsystems in addition to the microprocessor unit 28 are: (1)a collector/desorber assembly 30 employed to trap and then desorb vaporsfrom an air sample, (2) a cold spot concentrator 34 comprising one ormore series-connected cold spots used to successively concentrate vaporsdesorbed from a collector, (3) a short, high speed gas chromatograph 38for rapidly separating different substances, including vapors ofspecific compounds of interest, in the sample according to theircalibrated retention times; (4) a pyrolyzer unit 40 for decomposingselected vapors to produce nitric oxide gas (NO); and (5) a highlysensitive nitric oxide detector 42 which detects NO and produces signalsfor analysis and appropriate warning of the presence of selectedcompounds such as explosives. The construction and operation of each ofthe major subsystems is described in detail hereinafter in connectionwith detection of explosives, it being understood that many of theprinciples disclosed are applicable to detection of vapors of otherspecific compounds, particularly other nitrogen-containing compoundssuch as cocaine and heroin.

Collection/Desorption

A collector is included in the preferred detector system 26 chiefly toprovide increased sensitivity of the instrument and also to assist inseparating explosives vapors from potential interferents such asnitrogen oxides (NO_(x)) often present in air samples. Trapping ofvapors given off by, or stripped from, explosives (vapors which may bepresent at concentrations of one part in 10¹⁴ or lower) concentrates theavailable vapors so that, when combined with further focussing in coldspots, vapor concentrations are obtained which are sufficent fordetection in a nitric oxide detector. For example, an ozone-basedchemiluminescence analyzer employed as the preferred nitric oxidedetector 42 has a sensitivity level of about one picogram (10⁻¹² g.) ofNO, as defined by the amount of NO required to give a response fivetimes greater than the level of noise. This is equivalent to asensitivity to RDX of about 2.5 picograms of RDX. If 2.5 picograms ofRDX were decomposed in the pyrolyzer unit 40 to produce NO, the nitricoxide detector 42 would produce a response five times greater than thenoise level. To obtain RDX and other explosives vapors in amountssufficient for such detection, the detector system 26 employs a samplecollector as disclosed in the following paragraphs.

A key aspect of the collector/desorber assembly 30 is a surface which(1) will effectively extract or trap explosives vapors from a gassample, typically an air sample, as the sample is flowed over thesurface at a selected temperature such as room temperature; (2) willrepel or not effectively trap NO or NO₂ present in the gas sample, and(3) will release or desorb the explosives vapors without theirdecomposition when heated and under the flow of a gas such as hydrogen.A preferred surface is a coating of an organic silicone polymer such asa polymerized methyl silicone or polymerized methyl/phenyl siliconenormally employed in a gas chromatograph column. Such coating materialshave a three dimensional structure which entraps specific compounds suchas explosives by both chemical and physical means. This effect isdependent upon both the actual polymer being employed and thetemperature of the polymer.

One preferred collector 46, illustrated in FIGS. 2 and 3, is a cartridgecontaining a bundle of small diameter quartz tubes 50 whose innersurface is coated with a thin layer of the gas chromatograph (GC)material and which are closely packed in an annulus between an outerglass ring 54 and an inner glass piece 56. Application of mass transfertheory permits appropriate selection of tube diameter, tube length, andflow rates for high collection efficiency at manageably low pressuredrop. Tubes 50 having an inner diameter of about 0.53 mm, an outerdiameter of about 0.8 mm and a length of about 19 mm have been found tobe suitable, with flow and mass transfer calculations showing that at asample flow rate of about 130 cm³ per minute through a collectorcontaining 400 such tubes, at least about forty percent of the gasmolecules in the sample will contact the inner wall, and thus the GCmaterial 52, of such tubes. A typical tube bundle with about 400 tubes50 may have the tubes packed between a Pyrex glass ring 59 having anouter diameter of 32 mm and an inner diameter of 28 mm and a Pyrex glassinner piece or center support 56 with closed ends having an outerdiameter of 22 mm. Each tube 50 (see FIG. 4 showing a single tubeenlarged to illustrate various coatings) has a layer 52 of GC materialof thickness about 0.1 to 5 microns, typically 1-2 microns, on its innerwall. Preferred GC materials are a polymerized methyl silicone polymerknown as DB1 or a polymerized methyl silicone known as DB5, availablefrom J & W Scientific, of Folsom, Calif., and methyl/cyano silicones.The GFC material may be applied over a base coating of a dielectricmaterial such as silicon dioxide or silicon, preferably a thicknessabout 0.01-0.1 micron. The base or inner coating 53, which may not beneeded for coating quartz or glass tubes but may be required when the GCmaterials are applied to metal ribbons or metal tubes, as describedhereinafter, also acts as a wetting agent to promote adherence of the GClayer 52. A preferred method of applying the base coating 53 is to flowchlorosilane and toluene through the tube 50 and then flush withmethanol. The layer of GC material may be applied by aerosol spraying orby sealing one end of a tube, filling the tube with the liquid GCmaterial, and applying a vacuum.

The tubes 50 may also have a thin coating 58 of optical grade siliconeon their outer wall, to increase their strength and flexibility forbending, which may be useful in coiling the tube for application ofother coatings prior to cutting the tubes to size, and the same opticalgrade silicone material may be used to glue the tubes 50 in place duringassembly. The silicone coatings 58 on the outer wall of the tubes 50 maybe doped with a small amount (e.g., 0.5%) of carbon black to increaseabsorption of infrared radiation used to heat the GC material 52 duringdesorption of explosives vapors from it.

Alternate sample collectors utilizing arrays of tubes 62 are illustratedin FIGS. 5, and 6, and 7, which, for clarity, show portions ofcollectors in an intermediate stage of formation--that is, tubes packedfar less densely than the finished collector, and without stiffeners andradial support members which may be required in the collectors. Allthree arrangements include tubes 62, similar to the tubes 52, whoseouter walls are in contact with a thin metal foil so that each tube 62and the layer of GC material on its inner wall may be indirectly heatedby passing an electrical current through the foil. FIG. 5 shows acollector 70 with a metal foil 72 and an electrically insulating sheet74 wrapped in a spiral and sandwiching between them a spiral "line" ofcollector tubes 62. The foil may be formed of stainless steel or othersuitable metal about 0.01 mm in thickness with one side coated withadhesive to insure good thermal contact between the foil 72 and thetubes 62. The sheet 74 may be Teflon tape. Electrical leads 76 and 78are attached to both ends of the foil 72 so that a suitable power sourcemay be connected to the foil to deliver electric current and heat thefoil 72, in turn heating the layer of GC material to desorb explosivesvapors trapped by the layer 68.

In the sample collector 80 of FIG. 6 a metal foil 82 and insulatingsheet 84 are wrapped in a double spiral. This permits both electricalleads 86 and 88 to be positioned on the outside of the spiral-shapedcollector 80, facilitating connection of an electrical power source tothe collector 80.

FIG. 7 illustrates a sample collector 90 wherein a metal foil 92includes corrugations each shaped to closely fit around a portion of anindividual tube 62, with corrugated insulation 93 on the opposite sideof each tube. This provides a packing density of the tubes 62 somewhathigher than that of FIGS. 5 and 6. Packing density may also be increasedwith corrugated or uncorrugated foils--by arranging the tubes 62, whichare electrically insulating, so that they isolate successive turns ofthe foil, eliminating the need for an insulating sheet.

Sample collectors for the explosives detector system of the inventionmay, instead of utilizing electrically insulating tubes in thecollection of vapors, employ electrically conductive substrates coatedwith GC material. Such collectors may be less expensive to fabricatethan tube bundles, and they allow rapid desorption of sample vapors atrelatively low electrical power since their GC materials may readily beheated by passing an electrical current through the substrate. Onesuitable collector 94 with a metallic substrate, illustrated in FIG. 8,includes a metal ribbon 96 coated on both sides with a layer of GCmaterial formed into a high surface area, gas pervious structure such asa narrowly-spaced wound spiral which is mounted with a tube 100.Appropriate spacers such as non-conductive radial spokes 102 may beincluded and the ribbon may be corrugated to provide addition surfacearea. The ribbon 96, which is formed, for example, of molybdenum foilabout 0.002 cm thick, preferably has a base layer of dielectric materialof 0.01-0.1 microns thickness such as silicon dioxide or silicon appliedto both sides (as by sputtering, chemical vapor deposition, or othersuitable technique) prior to application of the GC material to promoteadhesion of the GC material and to prevent vapors trapped in the coatingfrom passing to the metal ribbon and tightly adhering to it. A suitablethickness for the GC material is 1-2 microns, and the GC materials maybe applied by aerosol spraying or other suitable technique. Explosivesvapors in air samples passed through the tube 100 contact the GCmaterial 98 and are trapped by it, and electrical resistance heating ofthe ribbon 96 subsequently releases the vapors for further processing inthe explosives detector system 26.

Another sample collector 104 with an electrically conductive substrate,shown in FIG. 9, comprises a porous fritted material such as nickelwhich is coated with a suitable GC material and which may be mounted ina metal tube 106. Collection of vapors occurs as a gas sample is pulledthrough the frit 108, and the frit 108 also filters particulate matterfrom the gas stream, thus preventing such particles from beingintroduced into downstream portions of the explosives detector system.The frit 108 may, if electrically conductive, be heated for subsequentdesorption of the vapors by passing an electrical current through it orthrough the tube 106 with which it is in contact. It may also bepossible to desorb the vapors by passing heated carrier gas through theporous frit.

The above-mentioned sample collectors may comprise a bundle ofidentically coated ribbons or frits. Alternatively, some tubes or aportion of the ribbon or frit may have coatings of different materialsand/or thicknesses as to selectively collect and release differentexplosives. For example, coatings of high polarity may be required totrap explosives having high vapor pressures (e.g. EGDN and NG), whilecoatings of lower polarity are adequate to trap explosives of low vaporpressure (e.g. TNT and RDX) which stick easily. Typically, the highpolarity coatings cannot be used exclusively, however, because duringsubsequent desorbtion such coatings may not release the low vaporpressure explosives unless heated to temperatures at which suchexplosives decompose. Because the high vapor pressure explosives willusually be present at much greater concentrations than low vaporpressure explosives, only a small portion of the collector (e.g., a fewpercent of the tubes in a bundle) need be tailored to collect vapors ofthe high vapor pressure explosives.

Any of the above-described collectors may be mounted within an air-tightcollection/desorbtion chamber which is connected to cold spotconcentrators and in turn to other portions of the explosives detectionsystem. In such an arrangement, for example, with the collectorinstalled in a booth for screening people for possession of explosivesor specific drugs, the collector remains in one position both while agas sample is directed through the collector for trapping of explosivesvapors, and thereafter while the collector is heated and a suitablecarrier gas is passed through it for desorption of explosives vapors.Alternatively, the collector may be mounted in a hand-held samplerduring sample collection and then transferred to a desorption chamberfor further processing in the remaining subsystems of the explosivesdetector system. The provision of a hand-held sampler or sample gunpermits sampling in remote areas which might otherwise be inaccessibleto the explosives detector system and allows sampling of vapors bylifting or stripping from objects (e.g., surfaces) as well as samplingof vapors which are airborne due to the vapor pressure of explosives.

A preferred hand-held sampler (FIGS. 10-11) is a portable gun 120 whichcontains rechargeable batteries within and near the rear of abarrel-like housing 124 and which may also be plugged into a suitablesource of electrical power. Attached to the housing 124 is a pistol-griphandle 126 with a trigger 128 to operate a blower 130 and also toactivate lamps 132 mounted in the inlet (FIG. 11) of the gun. A top ringhandle 134 is also provided to assist a user in holding the gun 120. Thefront end of the housing 124 terminates in a flared inlet 136 whichleads to a central bore 138 for receiving a sample collector assembly140, as is shown loaded within the gun 120 (FIG. 10). A preferredcollector assembly 140 includes a cartridge-like collector, such as thecoated glass tube bundle 46 of FIGS. 2 and 3, attached to a bodystructure 142 which facilitates use of the collector, particularly itsinsertion into, and removal from, the gun 120 and a collector/desorberassembly 30 (FIG. 12). The body structure 142 includes a hollow bodycartridge 144 to which the collector 46 is attached, as by gluing thecentral support 56 of the collector 46 to the cartridge 144, and lockingpins 146 and a spring-pin activator 148 are also attached to the bodycartridge 144 to permit loading of the collector assembly 140 within,and its removal from, the gun 120 and the collector/desorber assembly30.

The central bore 138 of the gun 120 communicates with a blower 130 whichdraws air samples through the collector 46, and it may be desirable,through not illustrated herein, to utilize the exhaust of the blower 130to form air jets which may be directed at a surface to aid in heatingthe surface and dislodging vapors from the surface. Extending into thebore 138 from the rear of the gun 120 is an extraction rod 150 which isused to move the collector assembly 140 into and out of the bore 138.For example, movement of the rod 150 within the hollow portion of thebody cartridge 144 and towards the inlet of the gun 120 causes the rodto push against an adjustable shoulder 152 extending through an endportion of the body cartridge 144 so that the collector assembly 140 ispushed out of the gune 120.

Operation of the blower 130 is controlled by the trigger 128 on thepistol-grip handle, which may be a double or two-position trigger topermit separate activation of the blower 130 and of the array of lamps132 mounted in the inlet 136 of the gun 120. The lamps 132, which maycomprise four gold-plated projection bulbs, aid in acquisition ofsamples. In particular, the lamps 132, when activated for a briefinterval of time such as 0.2 seconds near an object to be sample--e.g.,a surface contaminated with trace amounts of explosives--heat thesurface of the object which raises the vapor pressure of any explosivesthus heated and may also vaporize moisture or water containingexplosives particles. Action of the lamps 132, together with the blower130, desorbs and strips explosives vapors from the surface withoutdecomposing the vapors, and the explosives vapors are drawn into the gun120 and trapped within the collector 46.

With regard to heating provided by the lamps 132, other suitable meansfor heating surfaces include hot air jets, high speed flash lamps (whosevery rapid heating may avoid loss of heating by conduction by a metalsurface), irons which heat by direct contact, and microwave devices.Heating of explosives on a surface increases their vapor pressuresignificantly (e.g. up to a factor of ten for each 10° C. increase) andblowing onto, or locally vibrating the surface (e.g., ultrasonically),can help dislodge small particles of explosives which may be collected,then later desorbed as vapors.

It has been found during sample collection from surfaces that a "void"or inactive area exists directly underneath the collector--i.e., in linewith the central bore 138 of the sample gun 120--and that little or novapor may be collected from that inactive area. Essentially all of theair which is sucked into the gun is drawn along the target surface andthen into the collector 46 rather than into the collector from pointsdirectly below it. Hence, the gun 120 has been found much more effectivewhen used in an "asymmetric surface sampling" mode wherein the gun boreis aimed at an area immediately adjacent to the specific target area(e.g., a fingerprint) and the lamps 132 heat the target. Vapors then maybe drawn from the heated target area along the surfaces and up into thecollector 46.

After an air sample has been drawn through the sample gun 120 andexplosives vapors trapped in the collector 46, the collector istransferred to a desorption chamber 160 (FIG. 12) formed in thecollector/desorber assembly 30. To facilitate direct transfer of thecollector 46 from the sample gun 120 to the desorption chamber 160, thecollector/desorber assembly 30 may include a front mounting plate 164(FIG. 13). The plate 14 has an opening 166 to admit the collector 46 andcontains recesses 170 which mate with pins 174 protruding from the frontof the sample gun housing 124 so that the gun 120 may be locked to theplate 164 during transfer of the collector to the desorption chamber160. The plate 164 may have a saddle 175 extending from a lower portionthereof to support the gun 120 and may also include electrical contacts176 to permit recharging of batteries within the gun while the gun isattached to the plate 164. A slidable cover 178 is positioned behind thefront mounting plate 164 and may be moved by a lever handle 180 to blockthe opening 160 and seal the desorption chamber 160 after a collectorhas been unloaded from the sample gun 120.

As an alternative to transferring the collector cartridge from thesample gun to the desorption chamber 160, the gun and collector mayremain together and be loaded into the desorption chamber as a singleunit for desorption and futher processing of explosives vapors.Advantages of this arrangement may include a less complex design andfaster, more reliable operation.

Following transfer of a vapor-loaded collector 46 from the sample gun120 to the desorption chamber 160 (or the drawing of an air samplethrough the collector while the collector is positioned within thedesorption chamber 160), the GC material in which the explosives vaporsare held is rapidly heated to a suitable temperature, e.g. 160-200degrees C., to desorb the vapors from the collector for passage in acarrier gas to a cold spot 34 connected to the downstream end of thecollector/desorber assembly 30. With reference to FIG. 12, the carriergas may enter the desorbtion chamber 160 through holes or channelsformed in the front plate 184 and then pass through holes in the bodycartridge 144 to then flow through the collector 46. One method ofheating the GC materials is to preheat the carrier gas which is input tothe desorption chamber 160 from a suitable supply 182 (FIG. 16) and thenis directed through the collector 46 and into the cold spot 34. Thistechnique is suitable regardless of the type of material employed as asubstrate to which the GC materials of the collector has been applied,but may be rather inefficient and time- consuming. If a conductivesubstrate such as a metal ribbon or metal frit is used as a collectorsubstrate, then electrical resistive heating may be employed. If alight-transmissive collector is employed, such as a bundle of quartztubes, then high intensity infrared radiation is preferred for heatingthe GC material. The infrared radiation may, as illustrated in FIG. 13,be furnished by an array of lamps 184 such as eight 500-watt lampsspaced around the periphery of a 9 cm I.D., 15 cm long tube forming thedesorption chamber 160. Radiation from the lamps 184, whose absorptionmay be enhanced by use of a small amount (e.g., 0.5%) of carbon black inthe optical coating applied to the outside of each tube 50, rapidlyheats the collector tubes to a selected temperature in the range ofabout 160°-200° C., preferably 170° C. At these temperatures and duringa time period of about 20 seconds the explosives vapors are desorbedfrom the GC material of the collector without decomposition and areswept away by a carrier gas such as hydrogen flowing through the tubesand pass into a cold spot 34 connected to the collector/desorberassembly 30. The desorption also decomposes labile nitrogen compoundssuch as peroxyacetyl nitrate which might otherwise interfere withdetection of explosives. Any nitric oxide gas produced by suchdecomposition passes through the remaining portions of the detectorsystem 26 prior to the decomposition and detection of explosives.

It should be noted that the direction of flow of carrier gas through thecollector 46 during desorbtion is opposite to that of airflow duringsample collection. Such backflushing avoids having to push through theentire collector any dirt or other contaminants trapped near its inletduring sample collection.

Cold Spot Concentration

The second principal subsystem of the explosives detector system 26 is acold spot 34, or two or more cold spots of connected in series. Theseitems, termed "cold spots" because they are cooled and generally held ator below room temperature except during desorption events, function tofurther concentrate explosives vapors of a sample so that essentiallyall of such vapors may be input to a high speed gas chromatograph ofpredetermined capacity in a single "injection" of very small volumee.g., of about one to ten microliters. Each cold spot 34 preferablycomprises one or more metal tubes whose inner wall contains a layer ofGC material which may be similar to that employed in the samplecollector 46 and which may be applied over a base coating of siliconedioxide or other dielectric material. Alternatively, the metal tube ofeach cold spot may have threaded therethrough a section of gaschromatograph column coated with a GC material. The GC material, whenkept cool--e.g., at about 40° C. or less, preferably in the range ofabout -10° to 20° C.--removes and traps vapors of specific compoundssuch as explosives and the drugs cocaine and heroine from a mixture ofcarrier gas and vapors received from the collector/desorber assembly 30.Subsequent flash heating of the cold spot tube as carrier gas is passedthrough it desorbs the explosives vapors without their decomposition sothe vapors flow to the next cold spot and eventually to the analyticalcolumn of the high speed gas chromatograph 38.

A suitable construction for one stage of a cold spot 34 (FIG. 14)includes a stainless steel tube 188 of 1.6 mm outer diameter (OD) and0.99 mm inner diameter (ID) and having a section of 0.53 mm ID MegaboreGC tubing (available from J & W Scientific) threaded through it. The GCtube 191 may be lined with a monolayer of silicon dioxide (e.g. ofthickness about 0.0001 micron) and has a coating 192 of GC material ofthickness from 0.1 to 5 microns, typically 1-2 microns. The tube 191 mayhave a working length of about 10 cm thus an internal volume of about 22mm³.

A cold block 196 (FIG. 15) of aluminum surrounds the tube 188 and ismaintained at a constant temperature such as about 10° C. by a suitablecooler device such as a thermoelectric cooler 190, a commerciallyavailable, device utilizing semiconductor materials and without movingparts which provides cooling when an electrical current is passedthrough it. The portion of the tube 188 inside the cold block 196 iswrapped with one or more layers of insulation 198 such as siliconerubber, two layers being shown in FIG. 14, which isolate the tube 188electrically from the cold block 196. The insulation 198 also providesthermal resistance so that heat supplied to the tube, for desorption ofexplosives vapors will be conducted rapidly to the GC material 192 butonly slowly to the cold block 196. A thermocouple 193 may also beconnected to the tube to sense temperature for control duringdesorption.

A preferred method of flash-heating the cold spot tube 188 and thus theGC tube 191 to apply a voltage between its opposite ends and resistivelyheat the tube. For this purpose the ends of the tube 188 which extendfrom the cold block 196 may have nickel sleeves 195 and 197 crimpedaround them and connections made from the sleeves to a source ofelectrical power. Care must be exercised in constructing the cold spot,as in making sure the sleeves 195, 197 extend to or slightly within thecold block 196, so that the tube is rapidly and uniformly heated to thedesired temperature during desorption. Excess heating (hot spots) in thetube 188 could result in partial decomposition of certain explosiveswhile local cool regions can result in incomplete removal of explosives.Both alternatives are to be avoided since decreased sensitivity ofdetection would result.

The flash heater may be constructed and operated to initially apply arelatively high voltage across the tube 188 to quickly raise itstemperature and then after a preset time interval to switch to a lowervoltage so as to maintain the tube temperature at a desired level.Alternatively, a thermocouple such as the thermocouple 193 (FIG. 14) maybe attached to the tube 188 and interact with an appropriate controlcircuit to maintain predetermined temperatures. A third technique is tocontrol temperature based on measured resistance of the metal tube 188itself, as by applying AC or DC power to the tube 188 through a solidstate switch which is periodically opened for a short time to allow asmall sensing current to flow to the tube, and measuring the voltagedrop due to the sensing current. A temperature in the range of about150-200 degrees C., typically 170° C., has been found suitable fordesorbing explosives without decomposing then, the desired temperaturebeing a function of flow rate through the tube 191 and the specificvapors to be desorbed. After the heating and desorbtion of explosivesvapors into a carrier gas has been completed, e.g., over a time intervalof 0.01-15 seconds, the voltage applied to the tube 188 is discontinued,allowing the cold block 196 to cool the tube 188 and tube 191 down tothe cold block temperature for the next trapping of vapors in the tube191.

One or more additional cold spot tubes may be connected in series withthe cold spot tube 188, each successive cold spot tube preferably havinga smaller internal volume so as to provide progressively increasingconcentration of the vapors as they are moved from one cold spot tube tothe next. If two cold spots are used, the second cold spot may be asection of the analytical column of the gas chromatograph 38 threadedthrough a stainless steel tube.

FIGS. 15(a) and 15(b) show an end view and side view, respectively, oftwo cold spot tubes 188 and 203 which extend through a double cold blockassembly 194 which includes a cold block 196 and plates 197 and 199removably fastened to the block 196 by bolts 199. For clarity, the tubes188 and 203 are shown in FIG. 15 without their surrounding insulativewrappings, it being understood that the portions of both tubes 188 and203 which extend through the block 196 include wrappings similar tothose shown as 198 in FIG. 14. The tube 203, may have a GC tube 200 ofan inner diameter of about 0.32 mm and a working length of about 10 cmthreaded through it, and the GC tube 200 includes a suitable coating ofGC material which may be the same as that of the tube 191 of the firstcold spot. Tubes 188 and 203 are connected in series externally of thecold block 196, as by glass-lined stainless steel tubes, and each isconnected to a separate flash heater (not shown) to move explosivesvapors in successive steps from tube 191 to tube 200 and then into a gaschromatograph connected to the tube 190. Preferably the cold block 196,and tubing external to it, are enclosed in an oven 202 (see FIGS. 16(a),16(b) operated to maintain a temperature of about 150 degrees C. in thetubes external to the cold block so that explosives vapors do notcondense in the external tubing.

A schematic diagram of a system 210 suitable for rapidly and selectivelydetecting explosives vapors at high sensitivities (FIGS. 16(a) and 16(b)illustrates a preferred arrangement of flow lines and valving associatedwith two series-connected cold spots 212 and 214. In the system 210 thecold spot 212 and 214 may include the tubes 188 and 203 describedhereinabove, and the cold spots 212 and 214 are enclosed by the samecold block, which for simplicity is not shown in FIG. 16.

Flows of gas samples, carrier gas, and explosives vapors through thesystem 210 illustrated in FIG. 16 are regulated by variousmulti-position valves whose settings are controlled by a microprocessor28 (see FIG. 1) which also controls operation of other components of thesystem such as the various heaters used in desorbtion of explosivesvapors, vacuum pumps, pyrolyzers, the nitric oxide detector, and the gaschromatograph. As best shown in FIG. 16(b), a multi-port (e.g. 6-port)valve 220 regulates flow along flow lines 232 and 234 between acollector 222 which is contained within a collector/desorber chamber(FIG. 12) and a valve 228, which may vent the flow to atmosphere or passthe flow to a vacuum pump 310 along flow line 229. The valve 220 alsoregulates flow through the cold spots 212 and 214 along flow lines 236,238, and 240. The valve 220 is also connected to a source of carrier gas244 by a flow line 248, and the flow line 248 may extend through amulti-position valve (not shown) to permit selection of carrier gas ofdifferent pressures from the carrier gas source 244--e.g., pressuresranging from about 4-12 psig.

Hydrogen is preferred as the carrier gas both for desorption ofexplosives vapors from the collector and the cold spots and for carryingthe vapors and their decomposition products through components of thesystem 210 downstream of the cold spots 212 and 214. Hydrogen is readilyavailable for example, by electrolysis of water, and does not react withexplosives vapors or nitric oxide, and its low molecular weightfacilitates rapid flow through the system 210 and hence fast detectionof explosives. It also provides output signals with high resolution(sharp peaks) of output signals. The use of hydrogen rather than airavoids response of the detection system to nitrogen-containing compoundsother that those which contain the --NO or --NO₂ moiety since, withhydrogen, there in no oxygen available to convert such compounds to NO.Moreover, although hydrogen is reactive with air and more reactive yetwith ozone and thus would be thought by others to be unsuitable for usein an air sampling system and for carrying nitric oxide into a detectorto which ozone is also introduced as a reactant, use of hydrogen in thesystem disclosed herein has proven quite effective and trouble-free.This is believed due to the small amounts of hydrogen used relative tothe concentrations required for explosions and because the reactions inthe chemiluminescent NO detector occur at low pressures created by avacuum pump.

Air, though lacking the above described advantages of hydrogen, isreadily available and may be used for desorption of vapors from thecollector. However, a non-oxidizing carrier gas is needed for desorptionfrom at least the second the cold spot. Helium is an alternative tohydrogen, though it lacks hydrogen's very high diffusivity and is notgenerally readily manufacturable on site as needed.

The system of FIGS. 16(a) and 16(b) is used to move vapors of specificcompounds such as explosives from the collector 222 to the cold spot212, then to the cold spot 214 and thereafter into a gas chromatograph250 in a series of steps as follows. (It should be understood that, asmentioned earlier, the collector 222 may trap explosives vapors whilepositioned either in a collector/desorber chamber forming part of anoverall detection system having interconnected subsystems or whilemounted in a portable sample gun from which the collector can be removedand transferred to a desorber chamber.) First, with the valve 220positioned as shown in FIG. 16(a) to permit flow from the collector 222to the cold spot 212, the collector 222 is heated and hydrogen gas ispassed at a predetermined pressure (e.g., 5-9 psig) from the carrier gassource 244 along the flow line 248 and sweeps explosives vapors from thecollector 222. The vapors and hydrogen carrier gas pass along flow lines232 and 236 at a relatively moderate flow rate (e.g., 50 cm³ /minute)into the cold spot 212 (maintained at a low temperature by its coldblock). The cold spot 212 traps and concentrates the explosives vaporsand the carrier gas exhausts from the system 210 along the flow line234. Independently, carrier gas, also preferably hydrogen, from thesource 244 may be directed at a selected pressure (e.g. about 9-12 psig)along flow lines 249 and 240 to purge the cold spot 214, or, if thedetector system 210 is being used for analyzing samples in successionand the cold spot 214 already contains a concentrated sample, to sweepexplosives vapors from the cold spot 214 (as it is being flash-heated)into the gas chromatograph 250.

After explosives vapors have been moved from the collector 222 andconcentrated in the cold spot 212, the valve 220 is switched to theposition shown in FIG. 16(b) and the explosives vapors are desorbed fromthe cold spot 212 and further concentrated in the cold spot 214. Toaccomplish this the cold spot 212 is rapidly heated and hydrogen carriergas from the source 244 sweeps the explosives vapors from the cold spot212 into the smaller cold spot 214 whose relatively cold GC materialtraps and concentrates the vapors.

Gas Chromatograph

After vapors of specific compounds have been concentrated in the coldspot 214, the vapors are "fired" to the gas chromatograph 250 by a finaldesorption effected by rapid heating of the cold spot 214 while carriergas is flowed through it. The gas chromatograph 250 separates differentcompounds such as explosives one from another and from other compoundswhich could interfere with their detection, again without decomposingthe explosives. Preferably, the gas chromatograph 250 includes arelatively short analytical section of tube (typically a coiled tubeabout 12 feet long) and is a high-speed device i.e., explosives vaporsinjected into its inlet end are rapidly separated and various componentsemerge from its inlet according to specific calibrated retention times,with all explosives vapors chromatographed in less than about 25seconds. A suitable gas chromatograph tube is a quartz capillary tubeabout 12 feet long and 0.32 mm inner diameter and whose inner surface iscovered with a GC material which may be the same as that employed in thecollector 222 and the cold spots 212 and 214. The tube is enclosed in anoven 268 which maintains a uniform temperature such as about 170° C. inthe tube suitable for separation of the explosives vapors through thetube without decomposition of the vapors.

The use in the gas chromatograph 250 of (1) a small diameter tube withthin coatings, (2) hydrogen as a carrier gas (providing highdiffusivity), and (3) injections into the gas chromatograph (GC) of verysmall volumes of gas sample (due to the concentrating or prefocussingaction of the cold spots) results in fast gas chromatograph--i.e., gaschromatography which is completed in about 25 seconds to as low as about4 seconds. Additional speed, so as to effect a very fast GC time of onesecond or less, has been demonstrated by rapid temperature programmingof gas chromatograph tube--that is, a rapid, controlled heating of thetube so as to successively more various explosives of different vaporpressures through the GC tube as the temperature is ramped up orincreased in a prescribed manner. These fast, and very fast, gaschromatography techniques, which are discussed in more detailhereinafter relative to a detection system including twoseries-connected pyrolyzers, are applicable not only to detection ofexplosives vapors and of other compounds but also in analytical labprocedures requiring gas chromotography. They provide at least twoadvantages. First they permit analyses to be performed in seconds ratherthan the several minutes (e.g., 8-40 minutes) required previously for ananalysis which includes gas chromatographic separation. Second, theyreduce the peak width of signals produced in a detector downstream ofthe GC, which both (1) enhances selectivity (sharp, spike-like peaksbeing easier to distinguish that wide, spread-out peaks) and also (2)increases sensitivity of detection (since the peak amplitude increasesso as to maintain same overall signal area).

Pyrolyzer Unit

The output of the gas chromatograph 250 is passed into a pyrolyzer unit40 which functions to decompose explosives vapors to yield nitric oxidegas. A preferred pyrolyzer unit 274 (FIGS. 16(a) and 16(b)) includes twopyrolyzers 276 and 278 of similar size and flow capacity and which aredownstream of a heated flow splitter 280 which serves to divide theeffluent of the gas chromatograph 250 into substantially equal flows.The use of dual pyrolyzers, each operable at a different temperature,substantially enhances the selectivity of the explosives detector, aswill be explained hereinafter.

The pyrolyzer 276 is operated as a high temperature pyrolyzer, typicallyat temperatures in the range of about 700 to 1000 degrees C., typicallyabout 750 degrees C., while the pyrolyzer 278 is heated to substantiallylower temperatures--e.g., in the range of about 160 to 250 degrees C.,typically about 200 degrees C. Rather surprisingly, and of considerablebenefit in the selective detection of explosives, it has been found thatthe use of silver as a surface in the low temperature pyrolyzer 278, inthe presence of hydrogen, selectively produces nitric oxide gas fromnitrite esters and nitramines at temperatures considerably lower thanthose required to produce nitric oxide from other nitrogen-containingcompounds such as those with C--NO₂ or N--NO bonds. Because of this,nitrite esters and nitramines, which contain the structure R--N--NO₂ andR--O--NO (where R may be an organic radical or any other suitableelement or combination of elements) and include explosives such as EGDN,NG, PETN, and RDX, and HMX, can be detected in an NO detector afterdecomposition at temperatures of about 200° C. without interference fromother nitrogen-containing compounds (which do not decompose to produceNO at such low temperatures).

At the present time only silver appears to be able to promote pyrolysisof certain explosives at temperatures at or below 200° C. Metals such asgold, platinum, nickel, and molybdenum, though they reduce somewhat thetemperatures required for pyrolysis of explosives when compared withnon-metallic (quartz or ceramic) surfaces in a pyrolyzer, still mustoperate at temperatures in excess of 300° C. to decompose any explosivesto yield nitric oxide. At such higher temperatures, however,nitrosamines and other non-explosives are also pyrolyzed, increasing therisk of false signals.

Accordingly, the low temperature pyrolyzer 278 (FIG. 17) preferablycomprises a tube 282 of silver or has an inside surface which containssilver. A preferred tube is a silver tube about of 0.4 cm inner diameterand about 35 cm in length. The tube may also contain a coiled wire ofsilver to provide additional surface area, and a silver wire or silverfrit may be used in a quartz tube or ceramic tube so long as adequatesilver surface is available to contact samples passing through the tube.The tube 282 is coupled to a heater such as an electrical resistanceheater 284 which surrounds a working length (e.g. 15 cm) of the tube 282and maintains the tube 282 at a preselected temperature as sensed by acentrally located thermocouple 285, so that vapors passing through itare heated as they pass rapidly through the tube--i.e., in a fraction ofa second. The working portion of pyrolyzer 278 may be enclosed in analuminum housing 287 and a heated interface 288 may surround the inletend of the pyrolyzer tube 282 extending through an endwall of thehousing 287 so as to prevent sticking of vapors during their transportfrom the gas chromatograph 250 to the working portion of the pyrolyzer278.

The precise reactions by which silver promotes the selective productionof nitric oxide from nitrite esters and nitramines at temperaturesmarkedly lower than those required to produce nitric oxide from C--NO₂and N--NO compounds are not fully understood. It is hypothesized thatthe mechanism involves the chemisorption of the O--NO₂ or N--NO₂ portionof the nitrite ester or nitramine onto silver and a chemical reactionwhich produces silver nitrite according to reaction 1 below (for anitrite ester, where R is an organic group).

    R--O--NO.sub.2 +Ag→AgNO.sub.2 +R--O(chemisorbed)    (1)

The silver nitrate then decomposes in the presence of hydrogen toproduce nitric oxide, water, and elemental silver (reaction 2), and theremaining fragment of the nitramine or nitrite ester reacts withhydrogen to produce an alcohol or amine (reaction 3), as follows:

    AgNO.sub.2 +H.sub.2 →Ag+NO+H.sub.2 O                (2)

    H.sub.2 +2R--O→2R--OH                               (3)

Chemisorption of C--NO₂ compounds onto silver apparently does not occurbecause the carbon lacks a lone pair of electrons. N--NO compounds donot chemisorb because apparently a nitro (--NO₂) group is required forproper steric and electronic interaction with silver. The hightemperature pyrolyzer 276 includes a tube 286 of dimensions and flowcapacity similar to those of the low temperature pyrolyzer 278 but whichis formed of, or lined with, ceramic such as aluminum oxide. A quartztube may also be suitable. A heater 290 surronds the tube 286 and isoperable to maintain a temperature in the range of about 700 to 1000degrees C. of the tube 286 and its contents. At these elevatedtemperatures vapors of known explosives as well as vapors of manynitrogen-containing compounds such as nitrosamines, perfumes, etc.,will, if present, decompose to produce NO. Timewise separation of thevarious compounds, along with interference-free detection of nitriteesters and nitramines decomposed in the low temperature pyrolyzer 278,permits accurate identification of explosives vapors.

Nitric Oxide Detector

The nitric oxide detector 294 to which the output of a selectedpyrolyzer 276, 278, is directed may be any suitable NO analyzer of highsensitivity and high speed, and is preferably a high speed instrumentutilizing known principles of chemiluminescence, photoionization, orelectron capture. The detector which is currently most preferred is achemiluminescence-based analyzer similar to, but substantially upgradedfrom, the type disclosed in U.S. Pat. No. 3,996,002, whose disclosure isincorporated herein by this reference to that patent, and which is usedas a component of the Model 502 TEA Analyzer available from ThermedicsInc. of Woburn, Mass. The chemiluminescence nitric oxide detector 294(FIG. 18) includes a reaction chamber 296 into which is fed along lines298 and 300, respectively, decomposition products of either pyrolyzer276, 278, and ozone from a suitable supply such as an ozonator 302, aknown device which forms ozone by bombarding oxygen in air withelectrons to form atomic oxygen, some of which combines with molecularoxygen to form ozone. Reaction of the ozone and any nitric oxide presentin the output of the selected pyrolyzer 276 or 278 in the chamberproduces NO₂, an excited form of NO₂ which rapidly decays, emittingradiation in a narrow wavelength range such as 0.6-2.8 microns which isdetected by a photodetector or photondetector 304. The resulting signalsare fed to a microprocessor unit 306 which produces timewise signaltraces or chromatograms on a chart recorder, and the processed signalscan also be displayed and if desired used to trigger alarms upon thedetection of NO from explosives vapors. The reaction products togetherwith excess ozone and carrier gas, pass through an ozone scrubber 308and are exhausted through a vacuum pump 310.

To provide fast detection required for near real-time analyses by thedisclosed detection system the preferred chemiluminescence detector 294includes an NO/O₃ reaction chamber with a small volume and a shape whichallows reactants to flow quickly through the detector--i.e., as "plug"flow. A reaction chamber shaped to provide a small depth of gas viewedby its photodetector produces a reduced level of DC noise from emissionsof excited oxygen and ozone. A chamber with small internal wall surfacearea, and including polished wall surfaces, reduces collisions withsurfaces and thus the level of AC noise. Because signal level isunaffected by such modifications, the sensitivity of detection isincreased. The detector 294 also is operated under vacuum--e.g., at apressure of about 1-2 torr, which provides low effective volumes of thereaction chamber. Fast reaction times, as well as higher sensitivities,are also promoted by the delivery to the reaction chamber of highconcentrations of ozone, for example, by supplying ozone as the outputof two or more series-connected ozonators or of a single ozonatorthrough which the ozone output is recycled to increase ozoneconcentration. Tests of two and three ozonators connected in series haveshown large increases in sensitivity of the NO detector 294. Theabove-described combination of features, together with fast electronicsin the microprocessor unit 28 processing signals from the detector 294and controlling various components of the system, provides rapid,selective detection of nitric oxide at high sensitivities.

Operation of the System

Reference is made to FIGS. 16(a) and 16(b) for further discussion of theoperation of the pyrolyzer unit 274 and the NO detector 294. Sampleoutput from the gas chromatograph 250 is divided by the flow splitter280 into two substantially equal portions which pass through thepyrolyzers 276 and 278. With the system 210 in the "low temperature"mode (FIG. 16(a)), high speed (e.g. 5 millisecond) valves 312 and 316downstream of the pyrolyzers 276, 278 are set to dump the output of thehigh temperature pyrolyzer 276 through the vacuum pump 310 and to passthe output of the low temperature pyrolyzer 278 to the NO detector 294.Provided that the sample included nitrite esters and/or nitramines, theresulting chromatogram, sketched in FIG. 19, will include well-definedpeaks, each of which is identifiable, by prior calibration of explosivesvapors of known composition, as a specific explosive. Because of thecompounds separated by the gas chromatograph 250, only nitrite estersand nitramines decompose in the low temperature silver-containingpyrolyzer to produce NO at the low operating temperatures of thepyrolyzer 278, only a few well-separated peaks will be present. Thus theidentification of explosives is rather simple and clear.

It should be understood that the "low temperature" modes of operation ofthe system 210 are shown in FIGS. 16(a) and 16(b), respectively, onlybecause those two figures offer a convenient means of illustrating thetwo modes. FIGS. 16(a) and 16(b) are not intended to indicate that thesemodes occur at times coinciding with particular settings of the valve220 or of desorbtions from the collector 222 or cold spots 212, 214.Instead, the "low temperature" and "high temperature" modes are timed tooccur (by action of the microprocessor unit) at times preselected so asto pyrolyze vapors of interest following their gas chromatographicseparation.

When the valves 312 and 316 are switched to operate the pyrolyzer unit274 in the "high temperature" mode (FIG. 16(b)) the output of the lowtemperature pyrolyzer 278 is dumped through the vacuum pump 310 and theoutput of the high temperature pyrolyzer 276 is passed to the NOdetector 294 for analysis. The resulting chromatogram, sketched in FIG.20, will include the same peaks as were present in the "low temperature"chromatogram of FIG. 19 and may include additional peaks indicative ofnitrogen-containing compounds (e.g., nitrosamines, perfumes) whichdecompose to produce nitric oxide at the higher temperatures maintainedin the pyrolyzer 276. Certain of the additional peaks in FIG. 20 areidentifiable as explosives by their position in the chromatogramrelative to prior calibrations. The peaks for explosives previouslyidentified in FIG. 19 need not be separated from signals of othercompounds which could otherwise interfere with detection of explosives.

Test Data

FIGS. 21(a) through 21(q) are chromatograms from tests of mixtures ofknown nitrogen compounds, some of which were explosives, conducted tocheck the operation of the pyrolyzers 276 and 278. The samples wereinjected into a gas chromatograph, whose output was directed into aselected pyrolyzer and then analyzed in an NO detector 294. The gaschromatograph included a glass column about six meters in length andwith an inner diameter of 0.25 millimeters and a coating of DB5 material(J & W Scientific) of about one micron thickness, and which was enclosedin an oven maintaining a temperature of about 150 degrees C. Hydrogen ata supply pressure of 5 psig was used as a carrier gas and the pressurein the chemiluminescent reaction chamber of the NO detector was about 1torr.

FIG. 21(a) is a chromatogram resulting from testing of a samplecontaining 0.2 microliters (one picomole) of the explosives NG, DNT,TNT, PETN, and RDX and which was analyzed after passing through the gaschromatograph 250 and the low temperature pyrolyzer 278 which included asilver tube, and a CTR trap (see discussion later in this subsection ofthe traps 330, 332 (FIG. 16)). For the tests whose results are shown inFIGS. 21(a) through 21(i) the temperatures of a 6-inch heat zone of thepyrolyzer 278 and a 12-inch heat zone immediately downstream of the6-inch zone were held at about 165 and 180 degrees C., respectively.

The FIG. 21(a) chromatogram shows that NG, PETN, and RDX, the explosiveswhich are nitrite esters or nitramines, produced clear peaks and thuscan readily be detected and identifed by prior calibrations at therelatively low temperatures employed. DNT and TNT, which are C-nitrocompounds (i.e., have C--NO₂ bonds) produced no detectable signals,indicating, as expected, that they were not decomposed in the lowtemperature pyrolyzer to yield NO. Similar results (not shown) wereobtained in tests of these explosives at a somewhat lower temperature(successive heat zones were maintained at 149° C. and 148° C.).

Chromatograms of selected nitrogen compounds which are not explosives(FIGS. 21(b) through 21(i), such as various nitromusks which are presentin perfumes, and pyrolyzed at about 180 degrees C. and under the sameconditions as the above mentioned sample of five explosives, show noclearly identifiable signal peaks (initial peak at left side ofchromatogram is a transient "front"). The substances injected were asfollows: 21(b)--0.2 microliters (1440 ng) of musk tibetene in dimethylketone solvent (DMK); 21(c)--0.2 microliters (856 ng) musk ketone (DMK);21(d)--0.2 microliters (3600 ng) hydroxycitronella schiff base (DMK);21(e)--0.2 microliters (1320 ng) musk ambrette (DMK); 21(f)--0.2microliters (1020 ng) trifluralin (a herbicide) (DMK; 21 (g)--0.2microliters (1500 ng) moskene (DMK); 21(h)--0.2 microliters (1910 ng)musk xylol (DMK); 21(i)--0.2 microliters (1910 ng) musk xylol (DMK). Theabsence of peaks indicates that no detectable decomposition of thesecompounds to NO occurred in the low temperature pyrolyzer and hence thatthese compounds would not interfere with detection of explosives whichare nitrite esters or nitramines during time intervals wherein theoutput of the low temperature pyrolyzer was directed to the NO detector.

FIGS. 21(j) through 21(s) are chromatograms from tests of varioussamples passed through a quartz tube pyrolyzer about nine feet longheated to temperatures ranging from 300 degrees C. to 800 degrees C. Atpyrolyzer temperatures of 300 degrees C. (FIGS. 21(j) and 21(k)) and 450degrees C. (FIGS. 21(n) and 21(o)) little or no detectable responseoccurs for any of the explosives tested, (1 picomole each of NG, DNT,TNT, PETN, and RDX), with the only distinguishable peaks (for PETN andRDX) having amplitudes just slightly above the noise level. The contrastbetween this lack of response and the clear signals for certainexplosives at lower temperatures when silver is used in a pyrolyzerconfirms the efficiency of silver in promoting the decomposition ofselected compounds to yield NO. Also, the observable peaks in FIG. 21(m)for the mixture of nitrosamines pyrolyzed at 300 degrees C. (0.2microliters of nitrosodimethylamine, nitrosopyrolidine,nitrosomethylaniline, nitrosonornicotine, and4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone) and for the mixture ofperfumes (0.2 microliters of muskambrette, musk ketone, musk tibetene,musk xylol, moskene, hydroxycitronella, and schiff base) pyrolyzed at450 degrees C. (FIGS. 21(p) and 21(q)) demonstrate the potential forinterferences due to these compounds as pyrolyzer temperatures areincreased. No discernable peaks are noted in FIG. 21(l), a test of thesame mixture of perfumes in the quartz pyrolyzer operated at the lowertemperature of 300° C.) Thus at 800 degrees C., a temperature at whichall explosives (including DNT and TNT) have decomposed in a quartzpyrolyzer to yield NO (see FIGS. 21(r) and 21(s)) and which approximatesthose temperatures formerly required for analysis of all explosives bydecomposition to, and detection of, nitric oxide gas, there could bedifficulty distinguishing between the peaks of explosives and those ofnon-explosives which have similar retention times in the gaschromatograph. However, because several explosives (those which arenitrite esters and nitramines) can be readily detected afterdecomposition in the low temperature silver pyrolyzer 278, only arelatively few explosives need be distinguished from potentialinterferents in the chromatograms resulting from decompositions in thehigh temperature pyrolyzer. Thus the explosives of interest can beclearly identified by using the low and high temperature pyrolyzers insequence.

In addition to the selectivity provided by the collector, cold spots,gas chromatograph, and the pyrolyzers disclosed herein, furtheravoidence of interferents and hence enhanced selectivity to explosivesdetection may be furnished by traps 330 and 332 in the flow lines 336and 338 between the pyrolyzers 276, 278, and the valves 312, 316controlling output flow of these pyrolyzers (see FIG. 16). The traps 330and 332 are preferably cartridges containing a granular material such asan alumino-silicate molecular sieve material of selected pore size, andare effective to pass NO but trap organic compounds, chlorinatedsolvents, and sulfur compounds which could otherwise produce responsesin a chemiluminescence NO detector which would interfere with properdetection of NO. Suitable traps or filters are described in U.S. Pat.No. 4,301,114, whose disclosure is incorporated herein by this referenceto it, and are available as CTR traps from Thermedics Inc. of Woburn,Mass.

A preferred method of operating the detector system 26 to rapidly detectvapors of selected explosives of a sample is to monitor products of thelow temperature pyrolyzer 278, during time intervals corresponding tothose for which the explosives vapors emerging from the gaschromatograph 250 are known from prior calibrations to be onlynitramines, nitrite esters, and other explosives which decompose in thelow temperature pyrolyzer 278, and to detect products of the hightemperature pyrolyzer 276 for the remaining time. This is readilyaccomplished by appropriate switching of the valves 312 and 316 atselected times controlling their operation. For example in analysis of asample for the explosives EGDN, NG, DNT, TNT, PETN, and RDX, whichemerge from the gas chromatograph 250 in the above-recited order andproduce peaks on a chromatogram at specific known times, only DNT andTNT require pyrolysis in the high temperature pyrolyzer 276. Thus,sample analysis is performed by (a) initially operating in the lowtemperature mode to produce signals indicative of EGDN, then NG, ifpresent, (b) at a predetermined time switching the high speed valves 312and 316 to operate in the high temperature mode so as to detect DNT, theTNT, and (C) at a second predetermined time, switching the valves 312and 316 to obtain signals from PETN and RDX, if present in the sample.From the above-described sequence of operations a three-sectionchromatogram is produced, and the chromatogram provides signalinformation over essentially all of the 20-30 second time interval ofinterest--i.e., little or no information is lost in switching betweenhigh and low temperature modes because the valves 312, 316 operate athigh speeds (e.g., five milliseconds) and because equal portions of thesample are continually flowing through each pyrolyzer 276 and 278 evenwhen the products of one are exhausted without first being analyzed inthe NO detector 294. Low temperature operation for two of the threeportions of the chromatogram avoids possible interfering signals inthose portions from compounds which pyrolyze at temperaturea above theoperating temperature of the low temperature pyrolyzer 278.

Alternate Systems

FIG. 22 shows, in block diagram form, an alternative vapor detectionsystem 350 in which two pyrolyzers 352 and 354 are connected in seriesrather than in the parallel configuration illustrated in FIGS. 16(a) and16(b). The series arrangement, together with an additional selectiveelement or gas chromatograph 368 between the pyrolyzers 352 and 354provides selectivity by time-shifting signals from various compounds,including certain explosives and various other compounds which couldotherwise interfere with detection of explosives whose signal are notshifted. The system 350 avoids valve shifting between pyrolyzersrequired in a "parallel pyrolyzer" arrangement and is thus less complex.Because the flow is not split between two pyrolyzers, but the entireflow passes through both, the system also offers higher sensitivity thana comparable "parallel" system. Additional improvements in sensitivityresult from the very high speed operation permitted by this arrangement.This may be important in detecting vapors of explosives having low vaporpressures (e.g., plastics) and whose airborne concentrations may be aslow as one part in 10¹⁵ or 10¹⁶.

A gas sample which has been collected from the air is processed in thesystem 350 by being desorbed from a collector in collector/desorberassembly 362 then concentrated in a single cold spot or, if two coldspots 364 and 366 are employed, successively in cold spot 364, then coldspot 366. The desorbtion may be performed at or below atmosphericpressure by use of vacuum, which reduces the time and amount of carriergas (e.g., hydrogen) required, and, if so, a flow restrictor may beprovided between the carrier gas supply and the collector to assist incontrolling flow. The flash-heated output of cold spot 366 is separatedin the selective element or gas chromatograph 360, which may include a0.32 mm ID quartz GC tube 2-6 feet in length and coated with a 1-2micron coating of DB5 GC material, and is passed with its non-oxidizingcarrier gas (preferably hydrogen) into the low temperature pyrolyzer352. The low temperature pyrolyzer 352 may be a silver-containingpyrolyzer operable at a temperature in the range of about 160° to 250°C. so as to decompose nitramines and nitrite esters to produce NOwithout decomposing other explosives, perfumes, nitrosamines, etc.Alternatively the pyrolyzer 352 may be a non-silver pyrolyzer--e.g., aquartz pyrolyzer or nickel-containing pyrolyzer, operable at atemperature, such as about 275° C., sufficient to decompose nitraminesand nitrite esters to produce NO₂, again without decomposing otherexplosives, perfumes, nitrosamines, etc. Either pyrolyzer may be about10 cm in length and of small diameter--e.g., a quartz capillary tube of0.32 mm ID extending through electric-resistance-heated needle-stockstainless steel tube. The NO or NO₂ produced in the low temperaturepyrolyzer 352 passes through the gas chromatograph 368 very rapidly,essentially without being retarded at all by the active coating of itsanalytical column (which may be a 0.32 mm quartz GC tube 2-6 feet longand with a 1-2 micron coating of DB1 GC material). Other compounds,particularly perfumes, nitrosamines, explosives vapors, etc., aredelayed in passage in the gas chromatograph 368 according to theirGC-separation characteristics.

The high temperature pyrolyzer to which the output of the gaschromatograph 368 passes is preferably a quartz or ceramic pyrolyzeroperable at a temperature in the range of 700°-1000° C., typically about750° C. Essentially all explosives vapors which enter the hightemperature pyrolyzer 354, plus any NO₂ produced in the pyrolyzer 352and certain other nitrogen-containing compounds, are decomposed in thepyrolyzer 354 to yield NO. Detection of the NO in the nitric oxidedetector 370 allows specific compounds such as explosives to beidentified, as illustrated in FIG. 23, a sketch of typical outputsignals which would be produced by the system 350. FIG. 23 shows thatexplosives which are nitramines and nitrite esters (e.g., EGDN, NG,PETN, and RDX) and which were decomposed in the low temperaturepyrolyzer 322) produce peaks in the first portion of the chromatogram,while peaks from other explosives such as DNT and TNT and fromnon-explosives occur later in time since they have been delayed by thegas chromatograph 368 prior to pyrolysis. The time-shifted signals thusdo not interfere with identification of the nitramines and nitriteesters. The time-shifted peaks are also further separated from eachanother, which may facilitate distinguishing explosive DNT and TNT fromnon-explosives. Also, because the flow is not divided and hence the NOconcentration is not reduced when series-connected pyrolyzers areemployed, overall sensitivity of the system 350 is increased up to afactor of two over that of a "parallel pyrolyzer" system.

An important advantage of the series-connected pyrolyzer system 350 ofFIG. 22 is that it facilitates very fast analysis by gas chromatography,for detection not only of explosives but also of other compounds ofinterest. In tests of the system illustrated in FIG. 22, gaschromatographic analyses of explosives have been performed in as fast as3 seconds following the injection of vapors into the first gaschromatograph 360 by flash-heating or very rapid "firing" of cold spot366 (in about 10 milliseconds). That is detection of the nitric oxideproduced from all explosives in a sample is completed in the 3 secondinterval after vapors are introduced into the gas chromatograph 360.Such very fast analyses would be difficult, if not impossible, toachieve in a "parallel pyrolyzer" system even with high-speed valves forshifting between pyrolyzer outputs delivered to the NO detector. Onereason for this is that in the parallel arrangement at least a fewseconds of purge time are required after switching a valve in order tosweep the gas flow out of the flow lines between the valve and the NOdetector and to insure that a proper baseline is re-established and thatno transient spikes from the valve action are mistaken for signals ofvapors to be detected.

To achieve the very fast gas chromatograph (GC) times of 0.5 to 3seconds, or less, extremely fast injections are required from a coldspot concentrator upstream of a GC, and the GC must betemperature-programmed. For the system configuration of FIG. 22, thismeans an extremely fast injection from cold spot 366 and that both gaschromatographs 360 and 368 must have their temperatures rapidlyincreased during selected times (e.g., over intervals of about 10milliseconds) controlled by the microprocessor unit 374 to quicklyrelease vapors from the chromatographs in a known sequence according tothe characteristics of each vapor (i.e., more volatile vapors will bereleased by their GC coating at a lower temperature-hence earlier intime-than less volatile vapors). A suitable method for accomplishingsuch temperature programming is to construct gas chromatograph systemsby threading the analytical columns of gas chromatographs 360 and 368through low-mass needle-stock stainless steel tubing, surround thistubing by an air bath, or other means for cooling the tubing and toresistively flash-heat the tubing (while monitoring temperature bysensing the resistance of the metal tubing) to activate thechromatograph column. Such an arrangement permits the temperature of thechromatograph columns to be ramped-up about 100° C. from a base level ofabout 50°-100° C. in a few milliseconds so that each gas chromatographcompletes a chromatographic separation in less than one second. Asmentioned earlier, very fast gas chromatographic analyses such as thesenot only save time but also boost sensitivity of detection since thesame mass of each specific vapors is effectively squeezed into a peak ofmuch narrower width. It should also be understood that fast, or veryfast gas chromatographic analyses, e.g. of nitramines and nitriteesters, can be performed in a system having a single GC column in serieswith a cold spot concentrator/injector, pyrolyzer, and specific gas(e.g., NO) detector rather than the dual pyrolyzer, dual GC arrangementof FIG. 22.

Additional Data

FIGS. 24 and 25 are chromatograms of tests performed as "system checks"or calibrations on a system similar to the system 210 shown in FIGS.16(a) and 16(b). The signal attentuation factor for each run plotted is32 and the interval between time marks on the time axis is about tenseconds. The data in FIGS. 24(a), (b), and (c) is from analysis ofmixtures of five explosives injected into carrier gas flowing throughthe system and that of FIG. 25 is from test samples obtained by heatingpaper towels. System parameters and operating conditions for the testsare summarized in Table 1.

TABLE--1 SYSTEM PARAMETERS AND TEST CONDITIONS

Collector--400 quartz tubes, each 19 mm long and with 0.53 mm. I.D.Internal surface of tubes were coated with 1.5 micron thickness ofpolymerized methyl or methyl/phenyl silicone as follows: 95% of tubescoated with DB1, 5% with DB5.

Cold Spot Tubes (CS1 and CS2)--Each metal tube contained one quartz tubeof working length about 100 mm and with internal surface of quartz tubecoated with 1 micron thickness of DB5. CS1 had 0.53 mm ID, CS2 had 0.32mm ID. Cold spot tubes were maintained at 9° C. except duringflash-heating, when CS1 was heated to 152° C. and CS2 was heated to 176°C.

Pyrolyzers (CP and HP)--Each had tube of heated length about 150 mm and4 mm ID. Low temperature pyrolyzer (CP) was formed of silver andincluded a coiled silver wire within it. High temperature pyrolyzer (HP)was formed of aluminum oxide. CP operated at 200° C.; HP operated at750° C.

Gas Chromatograph (GC)--Gas chromatograph (GC) had a quartz tube oflength about 15 feet with a 0.32 mm ID, and internal surace of GC tubewas coated with 1 micron thickness of DB5. GC was operated at constanttemperature 176° C.

Nitric Oxide Detector--NO detector included ozone-basedchemiluminescence reaction chamber operated at pressure of 0.38 mm Hg.

TIME SUMMARY (Analysis with time switching of parallel pyrolyzers)

    ______________________________________                                        Time (Seconds)                                                                            Event                                                             ______________________________________                                        1           H.sub.2 purge through collector (no heat)                         11          Desorb collector (high lamp heat)                                 7           Desorb collector (low lamp heat)                                  1           Establish flow through (switched)                                             6 - port valve - "ready" interval                                 15          Desorb CS1 (flash-heat)                                           1           Establish flow through (switched 6 -                                          port-valve-"ready" interval                                       4           Desorb CS2 (flash-heat)                                           6.67        Time T.sub.1 (following initiation of CS2                                     flash-heat) at which valves between                                           pyrolyzers and NO detector switched so                                        that HP output detected (instead of                                           the CP output detected until valves                                           switched)                                                         8.70        Time T.sub.2 (after T.sub.1) at which valves                                  switched so that CP output again                                              detected                                                          45          Total time data acquisition equipment                                         on - monitoring for peaks                                         ______________________________________                                    

FIGS. 24 (a), (b), and (c) are results of tests wherein 1.5 microlitersof a 10 picomole/microliter mixture of NG, DNT, TNT, PETN, and RDX inacetone were injected into a stream of hydrogen carrier gas ahead of thesecond cold spot (CS2), followed by detection of the nitric oxideproduced after the sample was fired into the gas chromatograph,GC-separated, and pyrolyzed. FIG. 24(d) shows results of a "control"test of an air sample collected by the sample gun (15 second unheatedair sample at a flow through the collector of about 2 liters/sec). Theabsence of any sharp, identifiable peaks in 24(d) indicates noexplosives were present. FIG. 24(a), a chromatogram from anlysis of theoutput of the low temperature pyrolyzer, shows that NG, PETN, (nitriteesters) and RDX (a nitramine) were clearly detected in the lowtemperature, silver-containing pyrolyzer, while DNT and TNT were not.The approximate sensitivity of detection of PETN for these chromatogramswas about 250 picograms--based on detectability of signal having anamplitude twice that of the noise. All five explosives were detectedwhen the output of the high temperature pyrolyzer was analyzed in the NOdetector (FIG. 24(b), and these explosives were also clearly sensed(FIG. 24(c)) when the high temperature pyrolyzer was "switched in" forthat portion of the analysis (labeled HP) during which DNT and TNT wereexpected from prior calibrations. It is evident that the system readilydetected the five explosives injected. Also, the chromatograms of FIGS.24(b) and 24(c) are nearly alike; however, had substances such asperfumes or nitrosamines been included in the samples, additional peakswould have been present in the chromatogram from the high temperaturepyrolyzer 24(b), likely masking one or more of the peaks for NG, PETN,and RDX, and hence interfering with their detection.

FIG. 25(a) shows data similar to that of 24(c) for analysis of vaporscollected by drawing a sample through a collector in the hand-heldsample gun, the collector then being transferred to a desorbtion chamberin a portable "cart" containing other components of the detectionsystem. The sample was obtained by heating a paper towel which had beensprayed with a standard solution of the five explosives NG, PETN, RDX,TNT, and DNT, in acetone, the sprayed towel having been stored in asealed glass jar prior to removal for testing. Heating was performed byflashing lamps of the sample gun (while drawing air through a collectorin the gun) a total of eight times for about 1/2 second durations and atintervals of about 3/4 second, then drawing more air (without flashing)for about 5 more seconds. The total air sample was about 11 liters overan 11 second interval. All explosives on the towel were readilydetected, as indicated in FIG. 25(a).

FIG. 25(b) shows results obtained when a paper towel similar to that ofFIG. 25(a), but containing no explosives, was sampled. No identifiablepeaks were detected, confirming that the peaks of FIG. 25(a) were due tothe explosives sprayed onto the towel.

It should be understood that solutions of specific compounds to be usedfor calibrations or system checks can also be sprayed on any suitablesorbent material and sealed in various containers, e.g. a foil-linedplastic pouch. This facilitates system checks even in remote locationsand avoids having to inject liquids (by syringe) into the vapordetection system to check its operation.

The detection system and the various components described herein may beembodied in other specific forms. For example, the detector may include,or be operated to employ, a single low temperature pyrolyzer if it isdesired to monitor samples for the presence of nitrite esters andnitramines only. Also, the cold spot(s) may be omitted or bypassed ifsample collectors of low gas-holding volumes are utilized and increasedsample collection times are permitted in a particular detectionapplication. The present embodiments are, therefore, to be considered asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are to be embraced therein.

What is claimed is:
 1. A method of calibrating a detector of vapors ofspecific compounds comprising:applying a solution of one or more of saidspecific compounds to a sorbent material; heating said sorbent materialand collecting vapors of said specific compounds from said sorbentmaterial by drawing an air sample of said vapors through a collectorcontaining surfaces coated with a material effective to selectively trapand release said vapors; and analyzing in said detector the vaporstrapped in said collector to determine the response of said detector tothe specific compounds applied to said sorbent material.
 2. A method asin claim 1 wherein said heating and collecting step is performed by ahand-held sample gun containing a collector with surfaces coated with agas chromatograph material.
 3. A method as in claim 1 wherein saidsolution contains a mixture of specific nitrogen-containing organiccompounds, and said solution-applying step comprises spraying saidsolution onto a sorbent material.
 4. A method as in claim 3 wherein saidsorbent material is a paper towel.
 5. A method as in claim 1 including,between said solution-applying step and said heating and collectingstep, sealing said sorbent material in an air-tight container and thenunsealing said container just prior to the time of said heating step. 6.A method as is claim 1 wherein said heating and collecting step includesflash-heating said sorbent material with lamps of a hand-held sample gunpositioned near said sorbent material and drawing air through acollector within said gun.
 7. A method as in claim 6 including,following said heating and collecting step, transferring said collectorfrom said gun to a desorption chamber, desorbing vapors from saidcollector into a garrier gas, gas chromatographically-separating saidvapors, and analyzing the products of said gas chromatographicseparation.
 8. A method as in claim 1 wherein said solution-applyingstep comprises spraying a mixture of explosives onto a sorbent material.